The wonderfully weird alvarezsaurs

There’s a pretty exciting alvarezsaur paper in the works that’s due out very soon and so as a brief primer I thought it time for a quick post on the Musings. It’s going to be a big year for alvarezsaurs in China if all the papers come through in time, but the first is likely to be the best and that’s due in just a day or so.

Alvarezsaurs are maniraptoran theropods, that is they belong to the derived group that includes things like oviraptorosaurs, dromaeosaurs and of course, birds. While everything else in this group have noticeably long arms and hands, those of the alvarezsaurs are notably, even strikingly, reduced. Although the clade is pretty new (the first animal only being identified in the 1990s) it was a while before they were widely accepted as non-avian dinosaurs hence their exclusion from ‘The Dinosauria’ and inclusion in the Mesozoic birds tome ‘Above the Heads of Dinosaurs’, but their position now as maniraptorans seems pretty secure. They do indeed have several features which resemble birds such as the big sternum and very avian-like legs and as discussed previously, when fossils are pretty incomplete a few convergent characters can mask the true identity of a species or group, especially if you only have very derived and highly modified taxa present.

Jonah Choiniere with a typically small alvarezsaur - Mononykys.

Most alvarezsaurs are really quite small, the biggest were only about 2 m long (which even by the standards of the relatively small maniraptorans is about average) and the smallest were a fraction of this – more like 50 cm or so. In gross appearance and proportions they looked rather like the average ornithomimosaur (long neck, small head, long legs, medium tail) but again it’s the hands that really stand out (or not). While T. rex is famous for having just a couple of fingers and the derived ceratosaurs have very small hands (and indeed even the basal ones are quite small) they don’t have much on the alvarezsaurs. The arms are massively reduced and the fingers largely gone except for one which is massive and robust. In short they have one giant finger and claw, one or two other small digits and a short, but powerful arm. It’s quite a combination.

Ecologically the consensus (or at least best supported hypothesis) is that these animals were insectivores or perhaps even more specialist ant and termite eaters. This would fit well with the numerous but very small teeth seen in the few skull elements we have preserved and the very robust forelimbs and giant claw but on an otherwise reduced arm. Digging animals sport many of these same adaptations so it would suggest that alvarezsaurs (or at least some of them) were digging for their supper either in the ground or more probably in wood.

That’s it for now as this is supposed to be a short primer, more to come very soon.

It will be interesting to see the Jurassic alvarezsaurid, since I’ve been fairly convinced they’re arctometatarsalins until now. The TWG matrix and just about all others leave out almost all of Sereno’s arctometatarsalian features, so it’s no wonder it finds them in Maniraptora. But I hear this basal Chinese specimen supports a maniraptoran ancestry, so I’m intrigued.

I never explicitly mentioned any form of Jurassic alvarezsaur. Not sure where you are getting your ideas from. 😉

I said it was a big year for alvarezsaurs and China. I never mentioned any Jurassic alvarezsaur from China. Certainly not before any embargo. I’m *merely* alerting people to the fact that there is much to be said about alvarezsaurs. And *nothing else*. 😀

Mickey,
my analysis includes all Sereno’s “arctometatarsalian” characters (and all TWG characters), and places alvarezsaurids in the same position as Senter’s analysis (as basal maniraptorans, more derived than therizinosaurs)…

Patagonykus (not to be confused with the bird Patagopteryx) has a small lateral metacarpal preserved, but we don’t know how many phalanges it had on digits II and III or if either had unguals. The Shuvuuia holotype has a phalanx from digit II or III preserved, while another specimen described as Shuvuuia (but perhaps more closely related to Parvicursor- Longrich and Currie, 2009) preserves complete digits II and III with unguals on each. Mononykus is likely to have digits II and III too since it has articulations for them on its carpometacarpi.